Method for producing a cellulose pulp of selected fiber length and coarseness by a two-stage fractionation

Abstract

Cellulosic pulps of selected fiber morphology are disclosed having a coarseness less than a threshold coarseness level. The threshold coarseness level is a function of average fiber length. The cellulosic pulps are especially useful for producing paper structures such as tissue paper. A method for producing the cellulosic pulps is also disclosed.

What is claimed is: 
     
       1. A method of producing a cellulose pulp of selected morphology having an average fiber length between about 0.7 mm to about 1.1 mm, the method comprising the steps of: providing an aqueous slurry comprising wood pulp fibers;   providing a first fractionation stage comprising one of a length classification stage and centrifuging stage;   directing at least a portion of the slurry to form an input stream to the first fractionation stage;   processing the input stream to the first fractionation stage to provide an accepts stream of the first fractionation stage;   providing a second fractionation stage comprising the other of the length classification stage and the centrifuging stage;   directing at least a portion of the accepts stream from the first fractionation stage to provide an input stream from the first fractionation stage to the second fractionation stage;   processing the input stream to the second fractionation stage to provide an accepts stream of the second fractionation stage;   processing the input stream to the length classification stage to provide a length classification stage accepts stream having an average fiber length which is at least 20 percent less than the average fiber length of the rejects stream of the length classification stage; and   processing the input stream to the centrifuging stage to provide a centrifuging stage accepts stream having fibers with a normalized fiber coarseness at least 3 percent less than the normalized fiber coarseness of the fibers in the rejects stream of the centrifuging stage wherein the pulp produced in the accepts stream of the second fractionation stage has an average fiber length between about 0.7 mm to about 1.1 mm and a fiber coarseness that is related to the average fiber length by the relation:   C<(L).sup.0.3 +0.3     wherein C is the fiber coarseness measured in milligrams of fiber eight per 10 meters of fiber length.     
     
     
       2. The method recited in claim 1 comprising the step of processing the input stream to the centrifuging stage to provide a centrifuging stage accepts stream having fibers with a normalized fiber coarseness at least 10 percent less than the normalized fiber coarseness of the fibers in the rejects stream of the centrifuging stage. 
     
     
       3. The method of claim 1 wherein the first fractionation stage comprises the length classification stage. 
     
     
       4. The method of claim 1 wherein the first fractionation stage comprises the centrifuging stage. 
     
     
       5. The method of claim 1 comprising providing an aqueous slurry comprising mechanical pulp fibers. 
     
     
       6. The method of claim 1 comprising providing an aqueous slurry of recycled chemical pulp fibers. 
     
     
       7. The method of claim 6 comprising providing an aqueous slurry of recycled softwood and hardwood chemical pulp fibers, wherein the softwood fibers comprise from about thirty percent to about seventy percent of the fiber weight of the slurry. 
     
     
       8. The method of claim 1 comprising providing an aqueous slurry of softwood and hardwood fibers and processing at least a portion of the aqueous slurry in the first and second fractionation stages to provide an accepts stream of the second fractionation stage wherein softwood fibers comprise at least 10 percent of the fiber weight of the accepts stream of the second fractionation stage. 
     
     
       9. The method of claim 8 comprising processing the input stream to the length classification stage to provide a length classification stage accepts stream having an average fiber length which is at least 30 percent less than the average fiber length of the rejects stream of the length classification stage. 
     
     
       10. The method of claim 8 comprising processing the input stream to the length classification stage wherein the fiber weight of the accepts stream of the length classification stage is between 30 percent and 70 percent of the fiber weight of the input stream to the length classification stage. 
     
     
       11. The method of claim 10 comprising forming the accepts stream of the centrifuging stage wherein the fiber weight of the accepts stream of the centrifuging stage is between 30 percent and 70 percent of the fiber weight of the input stream to the centrifuging stage.